Summary

Beet western yellows virus (BWYV) is a common virus disease in NZ. Secondary fungal infections particularly by Alternaria and Stemphylium are often associated with BWYV.

 

Identification

Yellowing of the leaves between veins spreads within foliage. Leaves become thicker and brittle leading to death. Note that symptoms can be confused with nutrient deficiencies. Secondary interveinal necrosis also occurs as the plants age.

 

Importance

BWYV can cause major losses in fodder beet. In sugar beet in Canterbury, DM losses of up to 30% have been recorded. Large patches of plants in fields can be affected. BWYV reduces leaf quality, palatability and yield.

 

Spread

BWYV is primarily spread by aphids, mainly the green peach aphid. Aphids can acquire the virus after feeding on sap for 5–10 mins on infected plants but usually take an hour or more to become highly infectious. Most transmission takes place within 6–12 hrs after feeding starts, although aphids can retain the virus for up to 17 days.

 

Management

Because of the time it takes for virus infection to be transmitted by aphids, insecticide can be effective. Insecticidal seed dressings effectively control aphids in the plant establishment phase. This should be followed by close crop monitoring to detect any build-up in aphid numbers, and timely application of insecticide if needed.

Controlling weeds that can harbour aphids helps reduce the risk of infestations, together with aphid control in neighbouring crops, such as brassicas. Aphids from these sources can colonise beet crops and spread infection through the stand.

No good source of resistance is reported to be available for BMYV in fodder beet.

Aphid flights mainly occur in late spring (Nov to early Dec) and autumn from mid-April to late May. Thus delayed sowing and monitoring of crops in autumn can help manage this disease.